The present invention relates to an automatic frequency control circuit for correcting the influence of a frequency offset at a radio data communication terminal in a system of equalizing received signals by obtaining transmission path characteristics during a preamble period by a narrow-band modulation method such as a GMSK (Gaussian-filtered Minimum Shift Keying) modulation method.
In many cases, a synthesizer and crystal oscillator with high oscillation precision cannot be used to generate radio frequency signals under limitations on cost, size, and the like. The radio section generally performs automatic frequency control to allow communication while controlling the frequency offset between the transmitting and receiving sides.
FIG. 4 shows the arrangement of a receiver side in a conventional automatic frequency control circuit disclosed in Japanese Patent Laid-Open No. 10-163816.
In FIG. 4, the receiver comprises an antenna 41 for receiving radio waves from a transmitter, a bandpass filter 42 for extracting a signal of a necessary band from the received signal, an amplifier 43 for amplifying the signal output from the bandpass filter 42, a mixer 44 for mixing the output signal from the amplifier 43 and a local frequency signal into a predetermined frequency signal, an oscillator 45 for oscillating the local frequency signal and outputting it to the mixer 44, an amplifier 46 for amplifying the output signal from the mixer 44, a quadrature demodulation circuit made up of mixers 47 and 48 for quadrature-demodulating the received signal output from the amplifier 46 and outputting I and Q signals (baseband signals) having a phase difference of 90xc2x0, a 0xc2x0/90xc2x0 phase generator 49, and an oscillator 50, and A/D converters 51 and 52 for A/D-converting the I and Q signals made of real and imaginary part amplitude values that are output from the quadrature detection circuit.
The receiver further comprises, as an automatic frequency control circuit, a phase rotating unit 53 for rotating the phases of the demodulated outputs from the A/D converters 51 and 52 by a necessary amount, a phase difference detection circuit 54 for obtaining the phase difference between a current signal component and a signal component one period before the current signal component, an average value detection circuit 55 for integrating the phase difference output from the phase difference detection circuit 54 a predetermined number of times to obtain the average value, an integrating circuit 56 for integrating the average value output from the average value detection circuit 55 in units of symbols, and a vector transformation circuit 57 for transforming a signal output from the integrating circuit 56 into real and imaginary part amplitude values.
Phase control operation of the automatic frequency control circuit having this arrangement will be explained.
When pseudo noise (PN) signals of a predetermined pattern repetitively transmitted from the transmitter during the preamble period before the burst period are received by the antenna 41, they are quadrature-demodulated to output I and Q signals having a phase difference of 90xc2x0 from the A/D converters 51 and 52. The phase difference detection circuit 54 obtains, using a table, the phases of the I and Q signals made of real and imaginary part amplitude values from the A/D converters 51 and 52, and obtains the phase differences between the obtained phases and a phase obtained from a PN signal before one period. The phase difference detection circuit 54 repeats this phase difference detection operation a predetermined number of times.
The average value detection circuit 55 repetitively receives the phase differences of PN signals from the phase difference detection circuit 54 to calculate an average value xcex94xcex8. The calculated average value xcex94xcex8 is fixed during the burst period subsequent to the preamble period. The integrating circuit 56 integrates the average value xcex94xcex8 in units of symbols, and outputs the integrated value to the vector transformation circuit 57. The vector transformation circuit 57 transforms the output from the integrating circuit 56 into real and imaginary part amplitude values, and outputs them to the phase rotating unit 53. The phase rotating unit 53 synthesizes the outputs from the vector transformation circuit 57 with a received signal, thereby correcting the phase.
In this prior art, the phase difference of the PN signal is obtained every burst. However, under the frequency selective multipath fading environment, a phase difference for accurately correcting the frequency offset between the transmitter and receiver cannot be obtained every burst owing to distortion, noise, and the like.
It is an object of the present invention to provide an automatic frequency control circuit capable of calculating a phase difference for accurately correcting the frequency offset between the transmitter and receiver every burst even under the frequency selective multipath fading environment, and improving the error rate of demodulated data arising from the influence of the frequency offset.
To achieve the above object, according to the present invention, there is provided an automatic frequency control circuit in a receiver for receiving and quadrature-demodulating a multilevel quadrature-modulated signal transmitted during a preamble period and a data period subsequent to the preamble period, comprising phase difference detection means for obtaining a phase difference between pattern signals per period on the basis of a quadrature-demodulated pattern signal component and a pattern signal component one period before the quadrature-demodulated pattern signal component, the pattern signals being repetitively transmitted with the same pattern from a transmitter during the preamble period, average value calculation means for calculating an average value of first phase differences between the pattern signals repetitively output from the phase difference detection means, thereby obtaining a second phase difference in units of symbols in a current burst, weighting means for weighting the second phase difference from the average value detection means and a third phase difference up to a previous burst, thereby obtaining a fourth phase difference up to the current burst, and phase correction means for correcting a demodulated received signal on the basis of the fourth phase difference from the weighting means.